Sample acquisition and sample analysis are frequently performed as two disparate processes, since in situations that are not time critical, it is generally acceptable to take one or more samples, and to transport those samples to an analytical laboratory for analysis. Environmental air quality and water quality samples are often handled in such a fashion. However, there are many situations in which the ability to take an environmental air sample, and to analyze that sample immediately, are critical to health and safety. Measuring the air quality in poorly ventilated spaces such as mines, determining the presence of chemical and/or biological agents on the battlefield, or after an actual or suspected terrorist attack, are examples of situations in which sampling and analysis should be performed as quickly as possible, preferably by employing an integrated sampling and sensing apparatus that can provide and immediate indication of a life threatening substance in the environment.
There are many examples in the art of integrated sampling and detection apparatus. Dräger-Tubes, which are manufactured by Drägerwerk AG, Lübeck), are one well-known example of an integrated sampling and detection system. These devices are used to measure the concentration of specific gases and vapors in real time. Over 200 different Dräger tubes are available for measuring more than 500 different contaminants. The design and principle of operation of each Dräger-Tubes is the same in every case. A chemical reagent system is housed in an enclosed clear glass tube; and the reagent system reacts by changing color when exposed to a specific gas or vapor. The concentration of the substance is characterized by the length of discoloration within the tube and can be read off directly from a scale printed on the glass tube. Different amounts of air must be drawn through the tube, depending on the type and sensitivity of the reagent systems used. The volume of ambient air that must be drawn through the tube by a Dräger pump is stated on each tube. This prior art chemical sampling and detection system thus consists of a Dräger-Tube and a corresponding Dräger pump.
Other integrated air sampling and detection systems specifically designed to detect trace gases in air include electrochemical sensors for the measurement of gases such as CO, H2S, O2, Cl2, SO2, NO2 etc, infrared sensors for the measurement of CO2, CH4 or alkanes, and catalytic (pellistor) gas sensors for measuring flammable gases.
One drawback of the prior art integrated sampling and detection devices is that the target of interest is often present in the sampled environment in extremely small amounts. Acquiring a good sample of a reagent at low concentrations is problematic. Even if the sensor is capable of detecting minute levels of an agent of interest, without a high quality and readily obtainable representative sample, the capabilities of the sensing system are inefficiently utilized. Even worse, an inadequate sample of a hazardous material can cause a detection device to falsely indicate the absence of that material.
The difficulty in obtaining a good quality sample is particularly evident with respect to sampling for airborne particulates or aerosols. For example, aerosols comprising small droplets of liquid dispersed into air are not easily analyzed unless the aerosol materials are separated from the air and concentrated in a sample that can then be accurately analyzed. As used herein, the term “particulates” (and its singular form “particulate”) will be understood to include aerosols, liquids, solids, or semi-solids that are sufficiently small to be dispersed within and carried about in air or other gases and may include inorganic or organic chemicals, or living materials, e.g., bacterial cells or spores. Also, the term particulates refers to solids or semi-solids introduced into a liquid that is then dispersed within air as an aerosol mist so that the solids are carried within the liquid droplets comprising the aerosol mist.
Generally, it is difficult to identify materials comprising particulates entrained in a gaseous fluid unless the particulates can be collected and concentrated in a specimen suitable for analysis by separating them from the air or other gaseous fluid. One significant application in which extremely low levels of particulates need to be sampled and analyzed quickly is to provide combat troops with individual sampling units, that either include their own sensor, or which can be read in the field under combat conditions.
Particle impact devices are commonly used for collecting particulates from gaseous streams in which they are dispersed. These collectors “sweep” a large volume of air, and concentrate any particulates collected to provide a high quality representative sample. Several different types of particle impact collectors are available. Functionally, these particle impact collectors generally employ circuitous paths with many abrupt changes of direction along the passages through which a particulate-laden fluid flows. The particulates, being substantially more massive than the molecules of the fluid in which they are entrained, fail to negotiate the abrupt turns in these passages and are thus separated from the moving fluid stream, collecting on the surfaces that they impact. In the presently available types of particle impact collectors, there is generally a trade off between simplicity and efficiency.
Stationary impact collectors that employ a fan to force air against the impact surface are relatively simple, but are somewhat less efficient than would be desired. Rotating arm collectors are more efficient, yet are also more complex, in that they require both the rotating impact collector and a fan to be independently driven.
It would therefore be desirable to provide a simple and efficient particle impact collector that is capable of yielding a high quality representative sample of particulates or aerosols. Such a device is described in commonly assigned, co-pending U.S. patent application Ser. No. 09/265,619, entitled “Impact Particulate Collector Using A Rotary Impeller For Collecting Particulates And Moving A Fluid,” which was filed Mar. 10, 1999, the specification and drawings of which are hereby specifically incorporated herein by reference. It would further be desirable to integrate such a sampling device with a sensor to provide a portable system capable of rapidly detecting the presence of an agent of interest, so that the sample that is collected does not need to be sent to a laboratory facility for analysis. The prior art does not teach or suggest a portable integrated sampler and sensor unit that employs a rotary impeller used to both collect particulates and move a fluid through the unit.
Furthermore, it would desirable to optimize the design of such a rotary impact collector to enhance its performance. The prior art does not teach or suggest how such optimization can be achieved.